Phase 7: MIT license, portfolio README, Docker, cross-platform start

- LICENSE: MIT
- README: rewritten as portfolio-grade docs (features with code pointers,
  architecture, Docker/Windows/macOS quick starts, BYO-model table)
- Dockerfile (3-stage: SPA build, pip wheels, slim runtime) + compose with
  a /data volume; .dockerignore keeps secrets and local data out
- database.py: AIDND_DB_PATH env override so deployments can relocate the
  SQLite file; documented in backend/.env.example
- start.sh: macOS/Linux dev script with first-run setup

Verified locally: production SPA build served by the backend (deep links
OK), DB created at the override path. Docker image itself untested here
(Docker not installed); flagged in plan/07.

Co-Authored-By: Claude Fable 5 <noreply@anthropic.com>
Claude-Session: https://claude.ai/code/session_01KFsGHju9szibJJa2YJcdbg

backend/app/database.py

@@ -1,9 +1,18 @@
+import os
 from pathlib import Path
 
 from sqlalchemy import create_engine, event
 from sqlalchemy.orm import DeclarativeBase, sessionmaker
 
-DB_PATH = Path(__file__).resolve().parent.parent / "data.db"
+# AIDND_DB_PATH lets deployments (Docker volume, hosted disk) relocate the
+# database; default stays backend/data.db for local runs.
+_env_db_path = os.environ.get("AIDND_DB_PATH")
+DB_PATH = (
+    Path(_env_db_path).resolve()
+    if _env_db_path
+    else Path(__file__).resolve().parent.parent / "data.db"
+)
+DB_PATH.parent.mkdir(parents=True, exist_ok=True)
 
 engine = create_engine(
     f"sqlite:///{DB_PATH}",